1. Technical Field
The present invention relates to a telephoto lens of an inner focus type preferable for a photographic camera or an electronic still camera, particularly, a single-lens reflex camera or the like, particularly relates to a telephoto lens having a large aperture ratio and of an ultra telescopic type (for example, full aperture F-number of about 2.8, focal length of about 400 mm).
2. Related Art
In a related art, a large aperture ratio telephoto lens of an inner focus type in which an full aperture F-number is as bright as about 2.8 and a focal length is about 300 mm is supplied to the market as a telephoto lens which is comparatively compact, well-arranged, having excellent way of us, however, there is a strong market request for a large aperture ratio ultra telephoto lens having a longer focal length. Generally, a telephoto lens includes a positive lens group having a focal length shorter than a focal length of a total system, and the positive lens group is arranged on an object side. The telephoto lens also includes a rear group having a negative refracting power is arranged successive to the positive lens group, and a focal length is enlarged at the rear group so as to have a predetermined focal length. On the other hand, in recent years, from a necessity of contracting a close range in accordance with a request from the market of enlarging a photographic range, a main current is constituted by a telephoto lens of an inner focal type capable of shortening a focusing amount (refer to Patent Reference 1 (JP-A-6-201989) corresponding to U.S. Pat. No. 5,438,455, Patent Reference 2 (JP-A-8-327897) corresponding to U.S. Pat. No. 5,745,306, Patent Reference 3 (JP-A-9-325269) corresponding to U.S. Pat. No. 5,828,490, and Patent Reference 4 (JP-A-11-160617) corresponding to U.S. Pat. No. 6,115,188). In a related art, it is known that arranging a first group has a positive refracting power, a second lens group has a negative refracting power, and a third lens group has a positive refracting power in order from an object side and a focusing group is the second lens group. Further, it is known that at least a portion of the second lens group or the third lens group is moved for vibration isolation in a direction orthogonal to an optical axis. In a case of such a constitution, when a front group is constituted by the first lens group and the second lens group, a total of the front group is provided with an extremely weak refracting power. According to a large aperture ratio telephoto lens, at a front group, a first lens group is further divided into two positive sub groups and various aberration amounts successively generated at the first lens group are dispersed to reduce to thereby enable to constitute a lens system having a bright F value.
When the first lens group is constructed by a constitution of being divided into two positive sub groups, as is seen in, for Patent Reference 1, there is frequently constructed a constitution in which the positive refracting power of the first sub group arranged on the object side is made to be larger than the positive refracting power of the successive second sub group (or as is seen in Patent Reference 2, the refracting power of the first sub group and the refracting power of the second sub group are made to be substantially to the same degree). According thereto, it is one of important problems to shorten a lens total length in a specification of a telephoto lens and the constitution is a constitution most effective for achieving the object. For that object, in the background art, there is constructed a constitution of a lens shape for making a spherical aberration particularly having a high sensitivity as small as possible while pertinently selecting glass used such that anomalous dispersion glass is used for correcting a chromatic aberration in the first sub group having a stronger positive refracting power. On the other hand, the second sub group is provided with a positive refracting power less than that of the first sub group, so an amount of generating a spherical aberration is also small. Therefore, the second sub group is constituted by a shape mainly to correct Off-Axis aberrations such as a field curvature.
Meanwhile, in a case of designing a large aperture ratio/ultra telescopic type lens having, for example, an full aperture F-number of about 2.8 and a focal length of about 400 mm, it is conceivable to proportionally enlarge to use a telephoto lens having, for example, a focal length of about 300 mm which is designed to provide a high performance. However, by simply proportionally enlarging the telephoto lens of the background art, a diaphragm mechanism, a focusing mechanism, and a vibration isolation mechanism and the like cannot sufficiently be converged to be compact, there is constituted a lens having a large aperture diameter, increasing a weight of moving groups and having poor operability. In order to resolve such a problem, it is particularly necessary to retrain a diaphragm diameter produced by long focal point formation from being increased. In the case of the telephoto lens of the inner focus type, as moving groups for focusing or vibration isolation, groups before and after the diaphragm are frequently used. Therefore, by restraining the diaphragm diameter, the lens can be arranged compactly without enlarging a focusing system or a vibration isolation system arranged before and after the diaphragm. Therefore, it has been desired to develop a large aperture ratio/ultra telescopic lens capable of restraining a diaphragm diameter to be small and promoting operability in comparison with lens constitutions described in the respective patent references. For that object, it is necessary to devise a necessary condition different from that of the lens system of the background art. Particularly, it is conceivable to optimize constitutions of the first lens group and the second lens group from a view point different from that of the background art. For example, although according to the background art, as described above, the positive refracting power of the first sub group in the first lens group tends to be relatively increased, it is conceivable to constitute a distribution of refracting powers different therefrom.
The invention has been carried out in view of such a problem and it is an object thereof to provide a telephoto lens capable of providing a lens system advantageous for downsizing moving groups and suitable for large aperture ratio formation and ultra telescopic formation while maintaining an excellent optical performance from a long distance to a close range.